Dual tonometer pressure measurement device

ABSTRACT

An applanation tonometry system, for measuring pressure within an eye, the tonometry system comprising: an applanation tonometer device for providing specific applanation pressure to a plurality of locations on an eye; an electrical process controller for measuring pressure within the eye; and a tonometer hands free holder adapted to secure the tonometer device over the eye whilst measuring the pressure within the eye.

FIELD OF THE INVENTION

This invention relates to an apparatus for and a method of measuringintraocular pressure in the human eye. This invention more particularlyrelates to an applanation tonometer system for measuring pressure anddiurnal pressure variations within the eye .

BACKGROUND OF THE INVENTION

It is well known that excessive internal pressure within the humaneyeball is a component of glaucoma, a disease of the eye. This diseaseaccounts for a significant percentage of all blindness. Surveys haveshown it to be present and unrecognized in a significant number ofpeople, particularly people over the age of 40 and even more so forpeople over the age 50.

It is also known that where the presence of glaucoma can be identifiedat an early stage, damage to the eye and subsequent blindness can bearrested. Appropriate medication and surgery can serve to arrest theprogress of the disease so that useful vision is retained.

In view of the fact that glaucoma is widespread, numerous proposals havebeen made for measuring the internal eyeball pressure. Many of these arecomplex precision instruments, which are expensive, and which requireelaborate clinical settings for their operation. Typically, suchinstruments apply an amount of force to the eyeball, sufficient to allowan objective measurement of specific flattening (applanation) orindenting (indentation) of the surface of the eye. The amount of forcerequired to achieve a certain applanation or indentation is correlatedwith the intraocular pressure measured internally, and usually expressedin mm of mercury.

Conventionally, the clinical instrument involved has some element whichis applied directly to the cornea of the open eye to measure theapplanation or indentation of the cornea. In view of natural humanreflexes, this requires a topical anesthetic. The equipment is complexand costly and requires a trained and sophisticated technician tooperate it. Other proposals have been made, and the following patentslist proposals known to the applicant: U.S. Pat. Nos. 1,637,421;1,661,718; 2,656,715; 5,176,139; and 5,197,473; French Patent 2,542,603;and Russian Federation Patents 2,004,187 and 457,466.

The Lipschutz U.S. Pat. No. 1,637,421 is a pressure indicator. It is notconcerned with measuring eyeball pressure, but rather it is concernedwith applying pressure to other parts of the human body. It is based onthe well known phenomenon that sensitivity to pressure of an area of thebody is an indication of disease. More particularly, it relies on thefact that the progress of the disease is related to the sensitivity ofan associated area of the body. As such, it provides a device enablingthe pressure applied to a particular area to be measured, so thispressure can be correlated with the progress of the disease. No cleardirections are given, with regard to applying this technique to thehuman eye. Measuring pressure in the human eye presents unique anddifficult problems, as compared to other parts of the anatomy. As thehuman eye is sensitive and delicate, everyone has a strong, naturalreflex to close their eyes, if any attempt is made to touch the eye.This Lipschutz patent does not address this issue.

A hardness testing device is disclosed in U.S. Pat. No. 1,661,718 whichis of marginal relevance.

An ocular tension indicator is disclosed in the Tolman U.S. Pat. No.2,656,715. However, this requires the eyeball to be contacted. It reliesupon relative axial displacement of different components of known, setweight, to determine the pressure within the eye. As such, it appears tobe a delicate, precision instrument. Since it must contact the nakedeye, it cannot be used outside of a clinical setting.

The two Fedorov U.S. Pat. Nos. 5,176,139 and 5,197,473 disclose anocular tonometer and a related method. This relies on a somewhat uniquetechnique where a ball is permitted to fall freely onto aneyelid-covered cornea. The kinetic energy of the ball deforms a cornea.The amount of the ball rebound varies depending upon the amount ofintraocular pressure and this is judged against the height of the ballrebound. This technique would appear difficult to carry out, since itdepends upon judging the height of the ball rebound.

Russian Patent 457,466 discloses an intraocular pressure transducer.This relies upon a Hall effect generator. Weights determine thepenetration force of a plunger, whose displacement is sensed by the Halleffect generator with an output proportional to the displacement.Russian Patent 2,004,187 discloses an eye tonometer having a hollowcylindrical body with tips and working end face surfaces. It is notclear how this device is intended to work. In any event, it is againintended to be applied to the naked eyeball, which again would requirethe application of a topical anesthetic in a clinical setting.

U.S. Pat. No. 3,992,926 discloses an applanation tonometer incorporatinga transducer for generating a digital readout of pressure. In use, thetonometer device measures differential pressure changes during theintraocular pressure measurement process. The transducer identifies aspecific pressure change which corresponds with known tonograph results.

PCT patent application PCT/CA97/00341 discloses an applanation tonometerfor measuring intaocular pressure within the eye. The tonometer includesa main body and a plunger, wherein the plunger is brought up against theeyelid of a closed eye and the main body is pushed towards the eyelid,which displaces the plunger into the main body. Displacement of theplunger relative to the main body is terminated when a pressurephosphene is detected. The displacement of the plunger is determined bya marker which is also indicative of the intraocular pressure within theeye.

Now, one of the problems with measuring intraocular pressure is that itcan vary during the course of the day, and even from hour to hour.Accordingly, it is highly desirable to provide some simple technique formeasuring this pressure. This technique should enable an ordinary personto measure the intraocular pressure within their eyes, without requiringcomplex expensive equipment, without requiring attendance at a clinic orthe like, and without requiring the time of highly trained clinicalstaff. Also, it is highly desirable to record diurnal (occurring in a24-hour period) variations in intraocular pressure overnight. It hasbeen found that glaucoma patients have much higher diurnal intraocularpressure variations (8-11 mm Hg) in comparison to healthy subjects(about 4 mm Hg variations).

SUMMARY OF THE INVENTION

In an applanation tonometry system for measuring pressure within an eye,the system comprising: an applanation tonometer device for providingspecific applanation pressure to a plurality of locations on an eye; anelectrical process controller for measuring pressure within the eye; anda tonometer hands free holder adapted to secure the tonometer deviceover the eye. The tonometer device further comprises: a main body havingfirst and second end portion; a plurality of plunger members slidablymounted in the main body, each of the plurality of plunger membershaving a first and second end, the second end comprising a contactmember protruding from the first end portion of the main body, and asecond end mounted within the main body, in use, each contact memberapplies a specific applanation pressure to a specific location on theeye; and a plurality of transducer devices mounted within the main body,for converting the specific applanation pressure of each contact memberto an electrical signal.

In accordance with another aspect of the present invention, a method ofobtaining pressure within the eyeball of a subject comprises the stepsof applying a constant known reference pressure to a first location onthe eyelid of the subject; applying at least a second pressure to an atleast second location on the eyelid of the subject; increasing the atleast second pressure until an increase in the constant known referencepressure is detected; and terminating the increasing of the at leastsecond pressure, when the increase in the constant known referencepressure is detected.

The method of obtaining pressure within the eyeball of a subjectincludes providing a tonometer device comprising: a main body; a firstand second plunger member slidably mounted in the main body, the firstand second plunger member having a first end comprising a first and asecond contact member respectively and having a second end mountedwithin the main body, the second end of the first and second plungermember in contact with a first and second transducer devicerespectively, whereby the first transducer device detects displacementof the first plunger member and the second transducer device detectsdisplacement of the second plunger member, the method comprising:

(a) adjusting movement of the first plunger and the second plunger, sothe first plunger is in light contact with the first location on theeyelid, and the second contact member is in light contact with thesecond location on the eyelid;

(b) actuating the first plunger member so as to apply a constant knownapplanation pressure to the first location on the eyelid;

(c) actuating the second plunger member so as to apply an increasingsecond applanation pressure to the second location on the eyelid; and

(d) terminating the actuating movement of the second plunger member,when the first transducer device detects an increase in the constantknown applanation pressure.

DETAILED DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings, which show preferredembodiments of an applanation tonometry system of the present invention,and in which:

FIG. 1 illustrates a vertical cross sectional view of a first embodimentof an applanation tonometry system incorporating;

FIG. 2 illustrates a schematic view of the embodiment of each of thetransducer devices within the applanation tonometry system of FIG. 1;

FIG. 3 illustrates a block diagram representation of an electricalprocess controller incorporated within a tonometer device shown in FIG.1;

FIG. 4 illustrates the operating principle of the tonometry system shownin FIG. 1;

FIG. 5 illustrates a flow chart for the operation of a microcontrollerincorporated within the electrical process controller of the tonometerdevice; and

FIG. 6 illustrates a hands free holder for securing the tonometer deviceover a patient's eye, whilst the tonometer device is in use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a cross sectional view of an applanation tonometrysystem 10 comprising a tonometer hands free holder 12, an applanationtonometer device 11, first and second transducer devices 14 a, 14 b, andan electrical process controller 16. The tonometer device 11 has a mainbody 20, wherein the main body 20 has a first and second hand portion.

The first portion of the main body 20, as shown in the drawing, includesa housing 22 of increased cross section, where the cross section can beeither rectangular or circular. The housing 22 comprises an outer casing24 and a housing cavity 26 for holding the first and second transducerdevices 14 a, 14 b and the electrical process controller 16.

The second portion of the main body 20 defines first and second internalbores 30, 32 extending, in use, from the housing cavity 26 towards theeye (as detailed below). The cross section of each internal bore 30, 32is generally circular and is such that a respective first or secondplunger member 33, 34 is free to move axially therein.

The first plunger member 33 extends out of the second end of the mainbody 20 and is supported for sliding movement relative to the main body20 and within the bore 30. The sliding movement of the plunger 33 withinthe bore 30 is achieved by means of a bearing 38, located at the secondend of the main body 20.

The left hand end portion of the first plunger 33 includes a first heador contact member 40, which can be any desired shape. As illustrated inFIG. 1, the first contact member 40 is preferred to present a flat,circular disk surface, as indicated by 44, i.e. as a conventionalapplanation tonometer. Otherwise, the profile to the right of the disksurface, is not critical.

It is also to be noted that in contrast to conventional applanationtonometers intended for application directly to the eyeball, the head orcontact member is intended for application to the eyelid (althoughapplication directly to the eyeball is encompassed by the presentinvention). Accordingly, a larger applanation disk 44 may be required.The right hand end of the first plunger 33 is attached to a secondbearing member 48, which includes an annular channel 50 for receivingand retaining the right hand end of the first plunger 33. The bearingmember 48 further comprises an annular second bearing member, indicatedat 52. The length of the plunger extending between the first contactmember 40 and right hand end of the plunger is of uniform cross sectionand hollow. The first bearing member 38 and the second bearing member52, provide axial support for the first plunger 33 and allow the firstplunger 33 to move within the first bore 30 with reduced frictionalforce. This ensures that the frictional force between the plunger 33 andbore 30 is negligible so that any load applied to contact member 40 istransmitted through the first plunger 33 to the transducer device 14 a.

A first elongate frame member 54 is attached at one end to the secondbearing member 48 by means of a pair of securing members 56 a, 56 b. Theother end of the elongate frame member 54 extends axially along thefirst bore 30 and into the housing cavity 26 which holds the transducerdevices 14 a, 14 b and the electrical process controller 16. Thus, thecontact member 40, the plunger 33, the bearing member 48 and theelongate frame member 54 are integral and move as a unit within the bore30 and housing cavity 26. In use, pressure, as indicated by P1, isapplied to the right hand end of the elongate frame member 54 in thedirection of arrow F1. This causes an outward displacement of theelongate frame member 54 and first plunger 33 relative to the main body20. The outward displacement of the first plunger 33 in turn transfersthe pressure, indicated by P1, to the outer surface of an eyelid, asindicated at 43 a, by means of first contact member 40. Thus, it will beappreciated that in accordance with the present invention, the firstplunger member 33, the first elongate member 54 and first transducerdevice 14 a define a first pressure applicator for applying applanationpressure.

Similarly, a second elongate frame member 110 is attached at one end toa third bearing member 49 by means of a pair of securing members 59 a,59 b. The other end of the second elongate frame member 110 extendsaxially along the second bore 32 and into the housing cavity 26. Thesecond plunger 34 is also connected to the third bearing member 49.Identical to the first plunger member 33, the left hand end portion ofthe second plunger 34 (as viewed in FIG. 1) includes a second head orcontact member 46, which can be any desired shape. The plunger 33, thethird bearing member 49 and the second elongate frame member 110 areintegral and move as a unit within the second bore 32 and housing cavity26. In use, pressure, as indicated by P2, is applied to the right handend of the elongate frame member 110 in the direction of arrow F2. Thiscauses an outward displacement of the second elongate frame member 110and second plunger 34 relative to the main body 20. The outwarddisplacement of the second plunger 34 in turn transfers the pressure,indicated by P2, to a second surface on the eyelid, as indicated by 43b, by means of the second contact member 46. Thus, it will beappreciated that the second plunger member 34, the second transducerdevice 14 b and second elongate member 110 define a second pressureapplicator for applying applanation pressure.

Both the first and second pressure applicator are comprised ofsubstantially identical components and therefore, only the firstpressure applicator will be described in detail. Accordingly, thedescription for the first pressure applicator also applies to the secondpressure applicator. In accordance with the scope of the presentinvention, at least two pressure applicators apply applanation pressureto two separate locations on the eyelid.

The electrical process controller 16 within the housing cavity 26 isresponsible for controlling the actuation of actuator devices 68 and 74.Actuators 68 and 74 generate the outward displacement of the first andsecond plungers 33, 34, respectively, which provide applanation pressureto the surface of the eyelid 41. The electrical process controller 16monitors and controls the magnitude of applied pressure applied to thevarious locations on the eyelid 41, by means of the first and secondtransducer devices 14 a, 14 b. Furthermore, the electrical processcontroller 16 processes electrical signals received from both the firstand second transducer devices 14 a, 14 b in order to determine thepressure within a subject's eye.

The pressure applied by the first plunger 33 to the eyelid 41 is bothmeasured and generated by the first transducer device 14 a. The firsttransducer device 14 a comprises a first sensing device 60, a firstanalog-to-digital convertor (not shown in FIG. 1), a first actuatordevice 68 and a first coupling interface device 70. The first sensingdevice 60 has a sensor input and output, wherein the sensor inputreceives the pressure magnitude applied by the first plunger 33 to theeyelid 41. Based on the pressure magnitude received by the sensor input,the sensing device 60 generates an electrical pressure signal at thesensor output, which is in proportion to the pressure magnitude. Theanalog to digital converter receives the electrical pressure signal fromthe sensing device 60 and generates a digitized pressure signal. Theelectrical process controller 16 processes the received digitizedpressure signal generated by the analog to digital convertor. Based onthis processed digitized pressure signal, the electrical processcontroller 16 sends a first digital control signal to the actuatordevice 68 by means of conductor 72. Once the first actuator device 68receives the first digital control signal, it actuates the plunger 33 inorder to provide a specific or pre-determined applanation pressure tothe eyelid 41. As illustrated in FIG. 1, it is the linear displacementof the plunger 33 relative to the main body 20, which causes applanationpressure to the eyelid 41. The displacement of the plunger 33 isconverted to a pressure magnitude by the sensing device 60, whereby thepressure magnitude is correlated with a pre-determined pressure valuecorresponding to the linear displacement of the plunger 33. As theapplanation pressure applied by the plunger reaches the desired level,the digital control signal terminates the movement of the plunger 33.Consequently, the sensing device 60 and electrical process controller 16are implemented to form a closed-loop feedback control system whereby,the position of the plunger 33 is controlled based on a pre-determinedtarget pressure.

It will be appreciated that the analog to digital conversion may not berequired, especially if the electrical process controller 16 is capableof receiving analog electrical signals directly from the sensing device60. This would be the case if the electrical process controller wasimplemented as a custom designed mixed signal (analog and digital) ICchip. In applications where, for example, the process controller isimplemented using a standard off-the-shelf FPGA (Field Programmable GateArrary) device (with only digital inputs), the all digital inputs of theFPGA would require analog signals to be converted to digital formatprior to being received, as described in the previous paragraph.

The first actuator device 68 includes a first servo motor device 76having a rotatable shaft member 80 and an electrical motor controllerdevice 78. The motor controller device 78 drives and controls therotation movement of the motor device's shaft 80 based on the digitizedcontrol signal received from the electrical process controller 16.Similarly, the second actuator device 74 is identical in structuralcomponents to that of the first actuator device 68.

The first actuator device 68 is implemented using a servo-motorconfiguration. Therefore, the electrical motor controller device 78receives the digital control signal directly from the electrical processcontroller device 16 by means of conductor 72. The motor controllerdevice 78 receives the digital control signal and generates a PulseWidth Modulated Signal (PWM), which is sent by conductor 79 to the servomotor device 76. The PWM signal is a pulse train having a fixed period(e.g. 20 ms) and variable pulse width (e.g. 1-2 ms), which is used todrive the servo motor device 76. Based on the digitized control signal(e.g. 8-bit binary code corresponding to values between 0-255) receivedby the motor controller 78, the motor controller 78 generates a PWMsignal of specific pulse width value capable of varying the angularposition of the rotatable shaft 80 between 0-180 degrees. The angularposition of the shaft member 80 is normally initialized to a midrangeposition of 90 degrees by applying a 1.5 ms PWM signal to the motordevice 76. By varying the PWM signal between 1.5 ms and 1 ms, the shaft80 of motor device 76 will rotate between 90 and 0 degrees(anti-clockwise motion). Similarly, by varying the PWM signal between1.5 ms and 2 ms, the shaft 80 of motor device 76 will rotate between 90and 180 degrees (clockwise motion). Consequently, the motor controllercan incrementally vary the angular position of the shaft member 80 overa range of 90 degrees in either direction (clockwise andanti-clockwise), where each incremental degree of rotation can be in theregion of less than a degree.

The coupling interface includes a coupling input for receiving therotatable shaft member 80 of servo motor 76, and a coupling outputhaving a contact rod 82 adapted to engage the sensing device 60. Thecoupling interface device 70 couples the angular rotation of the shaft80 received by the coupling input into linear motion (forwards orbackwards) of the contact rod 82 at the coupling output (e.g. by use ofa screw mechanism). The contact rod 82 engages a first pressureapplanation plate 69, wherein the pressure applanation plate 69 is anintegral part of the elongate frame member 54. Therefore, as the shaft80 rotates, the linear motion of the contact rod 82 axially displacesthe pressure applanation plate 69, the elongate frame member 54 andconsequently the first plunger member 33.

Referring to FIGS. 1 and 2, the sensing device 60 comprises a firstpressure sensor IC device 64 and a first elastomer interface member 66having first and second opposing surfaces, as defined by 67 a and 67 b,respectively (see FIG. 2). The pressure sensor IC device 64 incudes apressure sensing surface and an electrical sensor output 96 (FIG. 2).The first surface 67 a of the elastomer member 66 is in contact with thepressure sensing surface of the pressure sensor device 64. The pressureapplanation plate 69, which forms part of the elongate frame member 54,is in contact with the second surface 67 b of the elastomer interfacemember 66. As the actuator device 68 displaces the contact rod 82, thepressure applicator plate 69 and the elongate frame member 54 outwards,as indicated by arrow F1, the pressure applicator plate 69 applies aspecific displacement to the surface 67 b of the elastomer interfacemember 66. This displacement generates a corresponding pressure withinthe elastomer interface 66 which in turn is applied to the first surface67 a. As the elastomer interface 66 is compressed against the surface ofthe pressure sensor IC 64, the sensor output of the pressure IC 64generates a corresponding electrical pressure signal in proportion tothe applied pressure.

Referring to FIG. 1, it is to be understood that in an alternativeembodiment of the present invention, the first pressure applicator plate69 is not attached to the elongate frame member 54 and only contacts thefirst surface 67 b (see FIG. 2) of the elastomer member 66. Also, thefirst sensing device circuit board 62, which includes the pressuresensor IC device 64 is then attached to the elongate frame member 54. Asthe actuator device 68 displaces the contact rod 82, the pressureapplicator plate 69 (which is now not an integral part of the elongateframe member) applies pressure to the elastomer member 66. This in turndisplaces the elastomer member, sensing device circuit board 62,elongate frame member 54 and plunger 33 outward towards the eyelid 41.As pressure is applied to the eyelid 41 by the first contact member 40,the elastomer member 66 is compressed between the pressure sensor ICdevice 64 (resisting the outward movement) and the actuated pressureapplicator plate 69 (moving outward). In this embodiment, the pressuresignal generated as a result of the elastomer member 66 compressedagainst the pressure sensor IC device 64 is directly correlated withapplanation pressure applied to the eyelid 41 by the first contactmember 40. Similarly, the second pressure applicator plate 112 would notthen be attached to the second elongate frame member 110 and onlycontacts the second surface 116 b (see FIG. 2) of the second elastomermember 114. Also, the second sensing device circuit board 106, whichincludes the second pressure sensor IC device 107 is attached to thesecond elongate frame member 110. The operation during pressureapplication is identical to that of the first plunger 33 devicedescribed in the paragraph.

As illustrated in FIG. 2, an electrical calibration device 98 receivesthe electrical pressure signal from the electrical output, indicated at96, of the pressure sensor device 64. If required, the calibrationdevice 98 generates a magnitude scaled (amplification or attenuation)version of the electrical pressure signal. The electrical pressuresignal is scaled during the calibration process of the tonometer device11. This is to ensure that the specific applied pressure measured by thepressure sensor device 64 correspond with the actual intraocularpressure within the eye. For this reason calibration is done againstmore sophisticated clinical tonometer devices such as the Goldmannapplanation tonometer.

The first sensing device 60 also comprises a first analog-to-digitalconverter 102 having an electrical input and an electrical output. Theanalog-to-digital converter 102 receives the scaled electrical pressuresignal at its electrical input, as indicated by 96, and converts thissignal to a first digitized pressure signal (DP1) at its electricaloutput, as indicated by 103. The first digitized pressure signal (DP1)generated at the analog-to-digital convertor output 103 is received atthe electrical process controller 16 by means of conductor 58 (see FIG.1).

As shown in FIG. 2, depending on the number of pressure applicatordevices used in the tonometer, each pressure applicator incorporates asensing device for measuring the applanation pressure applied by itscorresponding plunger member.

As shown in FIG. 1, a second sensing device 105 has a sensor input andoutput, wherein the sensor input receives a pressure magnitude inproportion to the pressure magnitude applied by the second plunger 34 tothe eyelid 41. Based on the pressure magnitude received at the sensorinput, the sensing device 105 generates a second electrical pressuresignal at the sensor output. The electrical pressure signal is sent bymeans of conductor 101 to the electrical process controller 16 forprocessing.

As illustrated in FIGS. 1 and 2, the second sensing device 105comprising a second pressure sensor IC device 107 and a second elastomerinterface member 114 having first and second opposing surfaces, asdefined by 116 a and 116 b, respectively (see FIG. 2). The secondpressure sensor IC device 107 also includes a pressure sensing surfaceand an electrical sensor output 115 (FIG. 2). The first surface 116 a ofthe elastomer member 114 is in contact with the pressure sensing surfaceof the pressure sensor device 107. A second pressure applanation plate112, which forms part of the second elongate frame member 110, is incontact with the second surface, defined by 116 b, of the elastomerinterface member 114. As the actuator device 74 (FIG. 1) displaces thepressure applicator plate 112, the elongate frame member 110 and asecond contact rod 122 outwards, as indicated by arrow F2, the pressureapplicator plate 112 applies a specific displacement to the secondsurface 116 b of the elastomer interface member 114. This displacementgenerates a corresponding pressure within the elastomer interface 114,which in turn is applied to the second surface 116 b. As the firstsurface, defined by 116 a, of the elastomer interface 114 is compressedagainst the surface of the pressure sensor IC 107, the sensor output ofthe second pressure sensor IC 107 generates a corresponding electricalpressure signal, which is in proportion to the specific applanationpressure applied by the elastomer interface to its pressure sensingsurface.

Referring to FIG. 2, a second electrical calibration device 118 receivesthe electrical pressure signal from the electrical output 115 of thepressure sensor device 107. If required, the calibration device 98generates a magnitude scaled (amplification or attenuation) version ofthe electrical pressure signal. The electrical pressure signal is scaledduring the calibration process of the tonometer device 11. This is toensure that the specific applied pressure measured by the secondpressure sensor device 107 corresponds with the actual intraocularpressure within the eye. For this reason calibration is done againstmore sophisticated clinical tonometer devices such as the Goldmannapplanation tonometer.

The second sensing device 105 also comprises a second analog-to-digitalconverter 123 having an electrical input and an electrical output. Thesecond analog-to-digital converter 123 receives the scaled electricalpressure signal at its electrical input, as indicated by 120, andconverts this signal to a second digitized pressure signal (DP2) at itsoutput, as indicated by 126. The second digitized pressure signal (DP2)generated at the output 126 is received at the electrical processcontroller 16 by means of conductor 101 (see FIG. 1).

As illustrated in FIG. 3, the electrical process controller 16 comprisesa firmware programmed microcontroller 130 (e.g. Philips 87LPC762), whichincludes on-chip memory for storing calibrated digital pressure valuesbased on the calibration of the tonometer device 11. The electricalprocess controller 16 maps the received first and second digitizedpressure signals (corresponds to the applied applanation pressure by thefirst and second plunger 33, 34) to the stored calibrated digitalpressure values in memory. The retrieved calibrated digital pressurevalues are used in subsequent processing steps (detailed in thefollowing paragraphs) for determining the intraocular pressure withinthe eye.

The microcontroller 130 comprises a plurality of digital pressure inputs132 for receiving digitized pressure signals from the each sensingdevice, such as sensing devices 60 and 105. If the tonometer 11 isimplemented with more than two plunger members, additional sensingdevices, such as sensing device 128 (shown in FIG. 2) will be added. Foreach added plunger member, additional sensing devices are required. As aconsequence, more digital pressure inputs are then required at themicrocontroller 130.

The microcontroller 130 also comprises a plurality of digital controloutputs 134 for controlling the actuation of each of the plungermembers. Based on the received first digitized pressure signal (DP1),indicated at 103, the microcontroller 130 generates a first digitalcontrol signal, as indicated at 128, which actuates the first plungermember accordingly. Similarly, based on the received second digitizedpressure signals (DP2), indicated at 126, the microcontroller 130generates a second digital control signal, as indicated at 136, whichactuates the second plunger member. An additional digital pressureinput, as indicated by 138, and the additional digital control output,as indicated by 140, can be utilized if the tonometer 11 includes athird plunger member.

Referring to FIGS. 1 and 3, it will be appreciated that in accordancewith the present invention, the first and second actuator devices 68, 74may both include a DC motor device, which is directly controlled bymeans of the electrical process controller 16. In this embodiment, afirst digital-to-analog convertor 129 converts the digital controloutput, defined by 128, generated by the electrical process controller16 into a first DC analog control signal, indicated at 133, that drivesa first DC motor. Similarly, a second digital-to-analog convertor 131converts the digital control output, indicated at 136, generated by theelectrical process controller 16 into a second analog DC control signal137 that drives a second DC motor. Both analog DC control signals 133,137 may require suitable amplification prior to driving the DC motors.

As previously mentioned, the tonometer can be adapted to use multipleplunger members, wherein each plunger member has a correspondingtransducer device. Each transducer device sends and receives pressureand control information from the microcontroller 130 in order to actuateits corresponding plunger member to provide applanation pressure to thesubject's eye. Each plunger member provides applanation pressure to aspecific region of the eye during the measurement of intraocularpressure. The microcontroller 130 also includes a digital readingoutput, as indicated by 142. Once the microcontroller 130 determines themagnitude of intraocular pressure, it generates a digitized pressurereading corresponding to intraocular pressure at the digital readingoutput, indicated by 142. A digital display device 145 receives thedigitized pressure reading from the digital reading output, indicated by142, and displays the digitized pressure reading corresponding tointraocular pressure.

It will be appreciated that in accordance with the present invention,the electrical process controller 16 can also be implemented using knownFPGA devices or as a custom fabricated ASIC (application specific IC).

As illustrated in FIG. 4, in use, the first plunger member 33 applies aconstant known reference pressure, defined by P′1, to a first locationon the surface of the patient or subject's eyelid, as defined by 43 a.The constant reference pressure is maintained on the eyelid 41, whilstthe second plunger 34 applies a steadily increasing second pressure,defined by P′2, to a second location on the subject's eyelid 43 b.

The steadily increasing second pressure is applied to a second locationon the subject's eyelid, as defined by 43 b, until a slight increase inthe known reference pressure is detected at the first location on thesubject's eyelid 43 a. During this step, the position of the firstplunger 33 is maintained fixed. When the slight increase in the knownreference pressure is detected, the application of the steadilyincreasing second pressure to the second location of the eyelid 43 b isterminated. The increase in known reference pressure as a result of thesteadily increasing second pressure, signifies the point at which thecombined affect of the second applied pressure and intraocular pressure,defined by P_(OC), within the eyeball just about overcome the appliedknown reference pressure. Therefore, the intraocular pressure isdetermined by a difference pressure, which is the pressure difference orpressure differential between the known reference pressure and secondpressure at the instance its steady increase is terminated.

As previously mentioned, the microcontroller device 130 (illustrated inFIG. 3) is responsible for controlling the actuation of the plungermembers and carrying out the required processing steps in order todetermine the intraocular pressure magnitude.

It will be appreciated that in accordance with the present invention,pressure is applied to a first location on the eyeball through theapplication of pressure to the first surface on the eyelid 43 a.Similarly, pressure is applied to a second location of the eyeballthrough the application of pressure through the second surface on theeyelid 43 b.

FIG. 5 illustrates a flow diagram representation of the functionalprocessing carried out by the microcontroller device 130. In a step 150,the position of the first contact member 40 relative to the subject'seyelid 41 is monitored in order to ensure that it is in light contact(negligible applied pressure) with the eyelid. This is an initializedposition for the first plunger 33. Referring to FIGS. 1 and 3, theinitialization position is achieved by the microcontroller 130 actuatingplunger member 33 so it is in light contact with the eyelid 41. Acontact sensor (not shown) incorporated into contact member 33 generatesa first electrical signal, as indicated by 143, which is activated as aresult of light contact with the patient or subjects eyelid 41. Once thecontact sensor is activated, the generated electrical signal, asindicated by 143, is received and processed by the microcontroller 130,wherein the microcontroller 130 terminates actuation of the plungermember 33.

Referring to FIG. 1, in achieving the initialized position, the firstplunger 33 can also be manually adjusted by the patient. A first manualadjustment mechanism 84 provides the means for manually adjusting thefirst plunger member 33 in order to achieve the initialized position.The manual adjustment mechanism 84 provides axial movement for the firstelongate member 54 and therefore the first plunger member 33 connectedto it. The manual adjustment mechanism 84 is screw threaded into theouter casing 24 of the right hand end of the housing, defined by 88.

As the adjustment mechanism 84 is turned clockwise, it engages the righthand end portion of the first elongate member 54, as defined by 90.Continued clockwise adjustment of the adjustment mechanism 84 pushes theelongate member 54 and first plunger 33 in the direction of thepatient's eye until light contact (negligible applanation pressure) ismade with the eyelid 41. The contact sensor or switch (not shown inFIG. 1) incorporated within the first contact member generates anaudible signal such as beep for indicating that the first plunger memberis in light contact with the eyelid and has been initialized. At thispoint, adjustment of the manual adjustment mechanism is stopped.

Once the first plunger 33 is in the initialized position (achievedmanually or by the microprocessor in step 150), a first initializationpressure signal is generated by the pressure sensor device 64. The firstinitialization pressure signal occurs as result of the first applanationplate 69 compressing the elastomer member 66 during the clockwiseadjustment of the manual adjustment mechanism 84. The firstinitialization pressure signal is converted to a first electricaloff-set pressure magnitude by means of the first sensor device 64 andthe analog-to-digital convertor 102 (FIG. 2). In a step 152, theelectrical off-set pressure magnitude is stored by the microcontroller130 (FIG. 3) and used in subsequent calculations of intraocular pressuremagnitude.

In a step 154, the position of the second contact member 42 relative tothe subject's eyelid is monitored in order to ensure that it is also inlight contact with the eyelid 41. This is an initialized position forthe second plunger member 34. Referring to FIGS. 1 and 3, theinitialization position is achieved by the microcontroller 130 actuatingsecond plunger member 34 so it is in light contact with the eyelid 41. Acontact sensor (not shown) incorporated into contact member 34 generatesa second electrical signal, as indicated by 144, which is activated as aresult of light contact with the patient or subjects eyelid 41. Once thecontact sensor is activated, the generated electrical signal, asindicated by 144, is received and processed by the microcontroller 130,wherein the microcontroller 130 terminates actuation of the secondplunger member 34.

Referring to FIG. 1, in achieving the initialized position, the secondplunger 34 can also be manually adjusted by the patient. A second manualadjustment mechanism 85 provides the means for manually adjusting thesecond plunger member 34 and achieving the initialized position. Themanual adjustment mechanism 85 provides axial movement for the secondelongate member 110 and therefore the second plunger member 34 connectedto it. As with the first manual adjustment mechanism 84, the secondmanual adjustment mechanism 85 is also screw threaded into the outercasing 24 of the right hand end of the housing 22.

As the second adjustment mechanism 85 is turned clockwise, it engagesthe right hand end portion of the second elongate member 110. Continuedclockwise adjustment of the second adjustment mechanism 85 pushes thesecond elongate member 110 and second plunger 34 in the direction of thepatient's eye until light contact (negligible applanation pressure) ismade with the eyelid 41. A contact sensor or switch (not shown inFIG. 1) incorporated within the second contact member 42 generates anaudible signal such as beep for indicating that the second plungermember is in light contact with the eyelid 41 and has been initialized.

Once the second plunger 34 is in the initialized position (achievedmanually or by the microprocessor in step 154), a second initializationpressure signal is generated by the pressure sensor device 105. Thesecond initialization pressure value occurs as a result of the secondapplanation plate 112 compressing the second elastomer member 114 duringthe clockwise adjustment of the second manual adjustment mechanism 85.The second initialization pressure signal is converted to a secondelectrical off-set pressure magnitude by means of the second pressuresensor 107 and the second analog-to-digital convertor 123 (FIG. 2).

In a step 156, the second electrical off-set pressure magnitude isstored by the microcontroller 130 (FIG. 3) and used in subsequentcalculations of intraocular pressure magnitude.

During adjustment of both the first and second manual adjustmentmechanisms 84, 85, the first and second applanation plates 69, 112disengage from the first and second contact rods 82, 122, respectively.Accordingly, the microprocessor 130 initiates an automated response byactuating the first and second contact rods 82, 122, such that theyengage the first and second applanation plates 69, 112. In essence, thecontact rods 82, 122 are catching up with the displaced applanationplates 69, 112 once the plungers 33, 34 and elongate members 54, 110 arein the initialized positions. As the contact rods 82, 122 engage theapplanation plates 69, 112, any additional pressure applied by thecontact rods 82, 122 will cause the applanation plates to furthercompress each elastomer member 66, 114. This will in turn cause thepressure sensors 64, 107 and analog-to-digital convertor devices 102,123 to generate higher pressure magnitudes than the stored first andsecond off-set pressure magnitudes determined in steps 152 and 156 (FIG.5). The microcontroller 130 then responds by actuating the motor devices76, 77 (FIG. 1) in order to reverse (back-off) the position of thecontact rods 82, 122, until the pressure sensors 64, 107 andanalog-to-digital convertor devices 102, 123 generate the same pressuremagnitudes as the previously stored first and second off-set pressuremagnitudes. The tonometry system 10 is now both mechanically andelectrically initialized.

In a step 158, the microcontroller device actuates the first plungermember 33 in order to apply a constant known reference pressure to afirst location on the subject's eyelid, which causes applanation of theeyeball. The first pressure sensor 64 and first analog-to-digitalconvertor 102 convert the constant known reference pressure to acorresponding reference pressure magnitude. The reference pressuremagnitude is received by the first digitized pressure input 103 (FIG. 3)of the microcontroller 130 and stored for processing. As the firstplunger 33 resumes the application of the constant known referencepressure to the eyelid (step 158), in a step 160, the microcontrolleractuates the second plunger member 34 to apply a steadily increasingsecond applanation pressure to a second location on the subject'seyelid.

In a step 162, the microcontroller continuously monitors the referencepressure magnitude, whilst continuing to increase the second applanationpressure by actuating the second plunger 34. Once a slight increase inthe reference pressure magnitude is detected by the microcontroller, ina step 164, the microcontroller terminates the actuation of the secondplunger 34.

The second pressure sensor 107 and second analog-to-digital convertor123 generate an electrical second pressure magnitude, which correspondsto the increasing second applanation pressure applied by the secondplunger 34. The second pressure magnitude is received by the seconddigitized pressure input 126 (FIG. 3) of the microcontroller. In a step166, the microcontroller stores the second pressure magnitude applied bythe second plunger 34 to the eyelid, once the actuation of the plunger34 is terminated, as defined in step 164.

The first off-set pressure magnitude, the second off-set pressuremagnitude, the reference pressure magnitude and the second pressuremagnitude obtained from steps 152, 156, 158 and 166 respectively, areprocessed in a step 168 by the microcontroller. In the step 168, themicrocontroller determines the difference between the first and secondoff-set pressure magnitudes. Both the first and second off-set pressuremagnitudes must be measured to be approximately the same, following theinitializing of the plunger members (steps 150 and 154). However, inuse, differences in component tolerances (in the sensing member) and thedifferences in the off-set pressure magnitudes as a result of the manualadjustment process, require the accountability of the off-set pressuremagnitudes in the calculation of intraocular pressure. Once thedifference between the first and second off-set pressure magnitudes havebeen generated, the microcontroller generates a differential pressuremagnitude by subtracting the second pressure magnitude from thereference pressure magnitude. A measure of intraocular pressure withinthe eye is determined by the differential pressure magnitude plus thedifference between the first and second off-set pressure magnitudes. Ifthe offset pressure magnitudes are approximately the same, then theintraocular pressure is defined by the differential pressure magnitude.

In a step 170, the calculated pressure magnitude is sent to the digitalreading output, as defined by 142 (FIG. 3) of the microprocessor 130.The digital display device 145 (FIG. 3) receives and displays thecalculated pressure magnitude corresponding to intraocular pressurewithin the eye.

As shown in FIG. 1, the hands free holder 12 comprises a cylindricalbody 175 and fastening means 180 a, 180 b. In use, the hands freeholder's fastening means 180 a, 180 b secure to wedged shaped grooves185 a, 185 b located on the main body 20 of the tonometer 11. Once thehands free holder 12 is secured to the tonometer 11, it can be strappedaround the back of the subject or patients head using elasticatedadjustable straps 188 a, 188 b in order to place the tonometer device 11over the eye. For this reason the left hand end portion of thecylindrical body 175 includes a pair of oval contact pads 190 a, 190 bfor contacting above and below the eye.

As illustrated in FIG. 6, the oval contact pads 190 a, 190 b arepositioned above and below the eye 200 and the straps 188 a, 188 b aresecured around the back of the patient's head. The elasticated nature ofthe straps 188 a, 188 b secures the hands free holder 12 and tonometer11 in a relatively perpendicular orientation to the patient's facialprofile. Once the hands free holder 12 and tonometer 11 are secured, thetonometer 11 can be angularly adjusted relative to the eyelid by meansof a pair of adjustment bearings 192 a, 192 b mounted within the mainbody 20 of the tonometer 11. A wedged shaped groove 185 a is providedwithin each adjustment bearing 192 a, 192 b. By applying a tilting force(in any given direction) to the right hand end portion of the tonometer11, the adjustment bearings 192 a, 192 b slide within the their mountedpositions within the main body 20.

It is desirable to record diurnal (occurring in a 24-hour period)variations in intraocular pressure overnight. It has been found thatglaucoma patients have much higher diurnal intraocular pressurevariations (8-11 mm Hg) in comparison to healthy subjects (about 4 mm Hgvariations). The Firmware in the microcontroller enables the tonometerdevice to make automated measurements whilst the patient is sleeping,which enables the measurement of diurnal variations in intraocularpressure overnight.

The embodiment of the present invention provides a differentialapplanation tonometry system with integrated electrical processingcircuitry for calculating intraocular pressure and variations inintraocular pressure. It should be understood that various modificationscan be made to the preferred and alternative embodiments described andillustrated herein, the scope of which is defined in the appendedclaims.

I claim:
 1. An applanation tonometry system for measuring pressurewithin the eye, the system comprising: (a) a tonometer securing devicefor securing the tonometer device in a substantial constant positionrelative to the eye, wherein the tonometer securing device includes amain body having a first end portion and a second end portion; and (b) aplurality of tonometer devices mounted to the tonometer securing devicefor measuring pressure at a plurality of locations on the eye, whereineach of the tonometer devices includes an applanation disk for measuringpressure within the eye by applanation tonometry through one of contactdirectly with an eye and contact with an eyelid, and wherein each of thetonometer devices further includes a plunger member slidably mounted inthe main body, each of the plurality of plunger members having a firstend and a second end, the second end including said applanation disk andprotruding from the first end portion of the main body and the first endbeing mounted within the main body and each tonometer device including atransducer mounted between one, respective plunger member and the mainbody.
 2. An applanation tonometry system as claimed in claim 1, whereineach transducer device is an electrical transducer for converting anapplanation pressure into an electrical signal.
 3. An applanationtonometry system as claimed in claim 36, wherein the plurality oftransducer devices each comprises: (a) a sensing device having an sensorinput and output, the input adapted to detect the magnitude of specificapplanation pressure from each contact member, and the output adapted togenerate an electrical pressure signal; and (b) an analog-to-digitalconvertor device comprising an analog input and a digital output, theanalog input adapted to receive the electrical pressure signal, and thedigital output adapted to generate a digitized pressure signal.
 4. Theapplanation tonometry system as claimed in claim 3, wherein theelectrical process controller comprises: (a) a plurality of digitalpressure inputs for receiving a plurality digitized pressure signalsfrom each of the plurality of transducer devices respectively; (b) adigital reading output for generating a digitized pressure readingcorresponding to intraocular pressure within the eye; and (c) aplurality of digital control outputs for generating a plurality ofdigital control signals corresponding to each of the plurality ofdigital pressure inputs.
 5. The applanation tonometry system as claimedin claim 4, wherein the plurality of transducer devices each furthercomprises a digital-to-analog convertor having a digital input and ananalog output, whereby the digital input receives one of the pluralityof digital control outputs and generates a corresponding analog controlsignal at the analog output.
 6. The applanation tonometry system asclaimed in claim 5, wherein each of the plurality of transducer devicesfurther comprises: an actuator device for providing linear slidablemovement of a plunger member, wherein the plunger member is one of theplurality of plunger members.
 7. The applanation tonometry system asclaimed in claim 6, wherein the actuator device includes: (a) a motordevice, and means for mounting the motor device within the main body,the motor device having a rotatable shaft member; (b) a couplinginterface device for coupling the rotatable shaft member to the plungermember; and (b) a motor controller device having a control input and acontrol output, the control input adapted to receive the analog controlsignal from the analog output of the digital-to-analog convertor, andthe control output adapted to generate rotation movement of therotatable shaft member.
 8. The applanation tonometry system as claimedin claim 6, wherein the actuator device includes: (a) a motor device,and means for mounting the motor device within the main body, the motordevice having a rotatable shaft member; (b) a coupling interface devicefor coupling the rotatable shaft member to the plunger member; and (b) amotor controller device having a control input and a control output, thecontrol input adapted to receive one of the plurality of digital controloutputs from the electrical process controller, and the control outputadapted to generate rotation movement of the rotatable shaft member. 9.The applanation tonometry system as claimed in claim 7 or 8, wherein thecoupling interface device includes a coupling input and a couplingoutput, the coupling input detecting the rotation movement of therotatable shaft member and the coupling output providing a linearslidable movement for the plunger member.
 10. The applanation tonometrysystem as claimed in claim 9, which includes a plurality of elongateframe members, each of the plurality of elongate frame memberslongitudinally extending between one of the plurality of plunger membersand the second end portion of the main body, the elongate frame memberseach having a first and second end, the first end attaching to one ofthe plurality of plunger members, and the second end located in closeproximity to a screw member, wherein the screw thread is threaded intothe second end portion of the main body.
 11. The applanation tonometrysystem as claimed in claim 10, wherein each of the elongate framemembers further includes an applanation pressure plate adapted to applya compression force to the sensing device.
 12. The applanation tanometrysystem as claimed in claim 11, wherein the coupling interface devicecomprises: a screw threaded barrel having a first end and a second end,the first end having a bore, the bore adapted to receive the rotatableshaft member, and the second end having a rod member for engaging theapplanation pressure plate; and a screw threaded housing adapted torotatably receive the screw threaded barrel, whereby rotation of theshaft member causes rotation of the screw threaded barrel, such thatlinear movement of the screw threaded barrel within the screw threadedhousing is provided.
 13. The applanation tonometry system as claimed inclaim 10, wherein the sensing device comprises: (a) an elastomerinterface member having a first and second surface, the first surface incontact with the applanation pressure plate, whereby the applanationpressure plate applies an applanation force to the first surface, whichis coupled to the second surface; and (b) a pressure sensor devicehaving a sensing surface and a sensor output, the sensing surface iscontact with the second surface, and the sensor output generating theelectrical pressure signal.
 14. The applanation tonometry system asclaimed in claim 13, wherein the electrical pressure signal correspondsto the magnitude of the specific applanation pressure.
 15. A method ofobtaining pressure within the eyeball of a subject, which includesproviding a tonometer device comprising: a main body; first and secondplunger members slidably mounted in the main body, the first and secondplunger members having a first end comprising a first and a secondcontact member respectively and having a second end mounted within themain body, the second end of the first and second plunger member incontact with a first and second transducer device respectively, wherebythe first transducer device detects displacement of the first plungermember and the second transducer device detects displacement of thesecond plunger member, the method comprising: (a) adjusting movement ofthe first plunger and the second plunger, so the first plunger is inlight contact with the first location on the eyelid, and the secondcontact member is in light contact with the second location on theeyelid; (b) actuating the first plunger member so as to apply a constantknown applanation pressure to the first location on the eyelid; (c)actuating the second plunger member so as to apply an increasing secondapplanation pressure to the second location on the eyelid; and (d)terminating the actuating movement of the second plunger member, whenthe first transducer device detects an increase in the constant knownapplanation pressure.
 16. A method as claimed in claim 15, wherein thefirst transducer generates a first electrical pressure signalcorresponding to the constant known applanation pressure; and the secondtransducer generates a second electrical pressure signal correspondingto the increasing second applanation pressure.
 17. A method as claimedin claim 16, which includes providing an electrical process controllerdevice comprising a first and second pressure input and a pressurereading output, the method comprising: (a) receiving the firstelectrical pressure signal at the first pressure input and generating areference pressure magnitude; (b) receiving the second electricalpressure signal at the second pressure input and generating a secondpressure magnitude; (c) generating an electrical differential pressuremagnitude by subtracting the second pressure magnitude from thereference pressure magnitude; and (d) sending the electricaldifferential pressure magnitude to the pressure reading output, when amagnitude increase in the reference pressure magnitude is detected dueto the increasing magnitude of the second pressure magnitude.
 18. Amethod as claimed in claim 15, wherein actuating the first plungermember provides the constant known applanation pressure to a firstlocation on the eyelid, which produces a first applanation pressure at afirst location on the eyeball.
 19. A method as claimed in claim 15,wherein actuating the second plunger member provides the increasingapplanation pressure to a second location on the eyelid, which producesa second applanation pressure at a second location on the eyeball.
 20. Amethod as claimed in claim 15, which includes providing a displaydevice, wherein the display device receives the electrical differentialpressure signal from the pressure reading output and displays a readingindicative of pressure within the eyeball.
 21. A method as claimed inclaim 20, wherein the electrical process controller is a microcontrollerdevice.
 22. An applanation tonometry system for measuring intraocularpressure within the eye, the system comprising: (a) a first pressureapplication device for applying a first, selected pressure at a firstlocation on the eye to cause deformation of the surface of the eye andincluding a first plunger member and a first transducer; (b) a secondpressure application device for applying a progressively increasing,known, second pressure at at least one second location on the eye, toincrease pressure within the eye and including a second plunger memberand a second transducer; and (c) a sensor for detecting when thepressure in the eye has been increased beyond the first, selectedpressure.
 23. An applanation tonometry system as claimed in claim 22,further comprising a securing device for securing the first and secondpressure application devices in a substantially constant positionrelative to the eye.
 24. An applanation tonometry system as claimed inclaim 22, further comprising a controller for controlling the secondpressure applied by the second pressure application device.
 25. Anapplanation tonometry system as claimed in claim 24, wherein thecontroller has an input for receiving a signal from the sensorindicating that the intraocular pressure is greater than the first,selected pressure.
 26. An applanation tonometry system as claimed inclaim 25, wherein the controller is adapted to stop the application ofpressure by the second pressure application device when the inputreceives a signal from the sensor indicating that the intraocularpressure is greater than the first, selected pressure.
 27. Anapplanation tonometry system as claimed in claim 22, wherein each of thefirst and second pressure application devices includes an applanationdisk for one of contact directly with the eye and contact with theeyelid.